Break a leg.
Good luck!
You can't teach an old dog new tricks.
It's hard for old people to learn something new.
Give it a shot.
Try it.
He got up on the wrong side of the bed.
He's grouchy.
Hitting the books.
Studying hard.
She is "catching some Z's."
She is sleeping.
The world is your oyster.
You can achieve anything.
He is feeling "under the weather."
He feels sick.
It's raining cats and dogs.
It's raining hard.
Multiplication is a "piece of cake."
It is not hard, it's easy!
You can say that again!
I agree!
Hit the sack.
Go to sleep.
In hot water.
In trouble.
Let's pig out!
Eat a lot!
You're pulling my leg.
You're joking.
Don't "throw in the towel!"
Don't give up!
It costs an arm and a leg.
It's expensive.
I'm all ears.
I'm listening.
You're the "teacher's pet."
The teacher's favorite student.
It slipped my mind.
I forgot.
Don't let the cat out of the bag.
It means don't tell the secret or don't ruin the surprise.
Hold your horses.
Wait.